Parasitic helminth infections in animals, including humans, are typically treated by chemical drugs, because there are essentially no efficacious vaccines available. One disadvantage with chemical drugs is that they must be administered often. For example, dogs susceptible to heartworm are typically treated monthly to maintain protective drug levels. Repeated administration of drugs to treat parasitic helminth infections, however, often leads to the development of resistant helminth strains that no longer respond to treatment. Furthermore, many of the chemical drugs cause harmful side effects in the animals being treated, and as larger doses become required due to the build up of resistance, the side effects become even greater. Moreover, a number of drugs only treat symptoms of a parasitic disease but are unable to prevent infection by the parasitic helminth.
It is particularly difficult to develop vaccines against parasitic helminth infections both because of the complexity of the parasite's life cycle and because, while administration of parasites or parasite antigens can lead to the production of a significant antibody response, the immune response is typically not sufficient to protect the animal against infection.
As an example of the complexity of parasitic helminths, the life cycle of D. immitis, the helminth that causes heartworm, includes a variety of life forms, each of which presents different targets, and challenges, for immunization. Adult forms of the parasite are quite large and preferentially inhabit the heart and pulmonary arteries of an animal. Sexually mature adults, after mating, produce microfilariae which traverse capillary beds and circulate in the vascular system of the dog. One method of demonstrating infection in the dog is to detect the circulating microfilariae.
If the dog is maintained in an insect-free environment, the life cycle of the parasite cannot progress. However, when microfilariae are ingested by the female mosquito during blood feeding on an infected dog, subsequent development of the microfilariae into larvae occurs in the mosquito. The microfilariae go through two larval stages (L1 and L2) and finally become mature third stage larvae (L3) which can then be transmitted back to the dog through the bite of the mosquito. It is this L3 stage, therefore, that accounts for the initial infection. As early as three days after infection, the L3 molt to the fourth larval (L4) stage, and subsequently to the fifth stage, or immature adults. The immature adults migrate to the heart and pulmonary arteries, where they mature and reproduce, thus producing the microfilariae in the blood. "Occult" infection with heartworm in dogs is defined as that wherein no microfilariae can be detected, but the existence of the adult heartworms can be determined through thoracic examination.
Heartworm not only is a major problem in dogs, which typically cannot even develop immunity upon infection (i.e., dogs can become reinfected even after being cured by chemotherapy), but is also becoming increasingly widespread in other companion animals, such as cats and ferrets. Heartworm infections have also been reported in humans. Other parasitic helminthic infections are also widespread, and all require better treatment, including a preventative vaccine program. O. volvulus, for example, causes onchocerciasis (also known as river blindness) in humans. Up to 50 million people throughout the world are reported to be infected with O. volvulus, with over a million being blinded due to infection.
Although many investigators have tried to develop vaccines based on specific antigens, it is well understood that the ability of an antigen to stimulate antibody production does not necessarily correlate with the ability of the antigen to stimulate an immune response capable of protecting an animal from infection, particularly in the case of parasitic helminths. Although a number of prominent antigens have been identified in several parasitic helminths, including in Dirofilaria and Onchocerca, there is yet to be an effective vaccine developed for any parasitic helminth.
As such, there remains a need to identify an efficacious composition that protects animals against diseases caused by parasitic helminths and that, preferably, also protects animals from infection by such helminths.
Macrophage migration inhibitory factors (MIFs), which are about 13 kilodaltons (kD) in size, have been identified in several mammalian and avian species; see, for example, Galat et al, 1993, Fed. Eur. Biochem. Soc. 319, 233-236, Wistow et al, 1993, Proc. Natl. Acad. Sci. USA 90, 1272-1275, Weiser et al, 1989, Proc. Natl. Acad. Sci. USA 86, 7522-7526, Bernhagen, et al, 1993, Nature 365, 756-759, Blocki et al, 1993, Protein Science 2, 2095-2102, and Blocki et al, 1992, Nature 360, 269-270. Although MIF was first characterized as being able to block macrophage migration, MIF also appears to effect macrophage-macrophage adherence; induce macrophage to express interleukin-1-beta, interleukin-6, and tumor necrosis factor alpha; up-regulate HLA--DR; increase nitric oxide synthase and nitric oxide concentrations; and activate macrophage to kill Leishmania donovani tumor cells and inhibit Mycoplasma avium growth, by a mechanism different from that effected by interferon-gamma. In addition to its potential role as an immunoevasive molecule, MIF can act as an immunoadjuvant when given with bovine serum albumin or HIV gp120 in incomplete Freunds or liposomes, eliciting antigen induced proliferation comparable to that of complete Freunds.
MIF appears to be related to glutathione S-transferase (GST) since at least some MIFs have GST activity and are able to bind to glutathione. MIFS, however, are only about half the size of GST subunits and do not show activity against 1-chloro-2,4-dinitrobenzene, which is the most common substrate used to detect GST activity. Although GST activity has been identified in several nematodes, that activity was detected using 1-chloro-2,4-dinitrobenzene, and the enzymes responsible for the activity were not of the size expected for MIFs. To the inventors' knowledge MIF homologues have not yet been identified in any parasitic helminth; efforts to do so have so far proven unsuccessful.